Trace element fractionation processes in resuspended mineral aerosols extracted from Australian continental surface materials
2008
Moreno, Teresa | Amato, F (Fulvio) | Querol, X (Xavier) | Alastuey, A (Andrés) | Gibbons, Wes
Unconsolidated surface soil and dust samples of varying trace element (TE) content were collected from remote locations in central and south-eastern Australia. The finer grained fraction of the samples (<10 μm, PM₁₀) was separated and geochemically compared to the parent particulate matter (PMPAR). TE are mostly hosted in phosphates and oxides/hydroxides or adsorbed to clay minerals, and are normally fractionated into the PM₁₀, producing PM₁₀/PMPAR ratios >1, especially in siliceous, TE-depleted dusts. In contrast, samples TE-enriched by primary silicate minerals eroded from igneous and metamorphic rocks can produce PM₁₀/PMPAR <1 for more mobile elements such as K, Na, Ba, Rb, and Sr. K/Rb is normally lower in PM₁₀ (unless the PMPAR is muscovite-rich) as is the light/heavy rare earth elements (LREE/HREE) ratio because both Rb and HREE are preferentially adsorbed by fine clay particles. Zr and Hf are mostly hosted by zircon crystals initially >10 μm but these diminish in size with time and sedimentological transport so that PM₁₀ aerosol concentrations of these elements are typically telescoped into a narrower range than the PMPAR. Nb is strongly fractionated into PM₁₀, with Nb/TiO₂ ratios characteristic of the durable host mineral rutile in all but the most TE-enriched PM. TE content of PM₁₀ in continental dusts is controlled by both physical and chemical processes. Fresh primary silicates suppress PM₁₀/PMPAR ratios of TE with low ionic potential, whereas the opposite effect is induced by hydraulic sorting and/or physical attrition during surface transport, as well as clay absorbtion and fixation of TE in small, resistant accessory minerals.
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